folder/add - Creates a new folder.

Method parameters:
name - The name of the folder.
parent_id - The ID of the parent folder.
auto_rename - Give the folder a unique name if a folder with the same name already exists.

Returns a JSON document of the following format:
<number> - The ID of the newly created folder.

Expected errors:
PARENT_FOLDER_NOT_FOUND - A parent folder with the ID doesn't exist.
ENTER_NAME - The name is empty.
FOLDER_ALREADY_EXISTS - A folder with the name already exists.